A typical electric power cable generally comprises one or more electrical conductors in a cable core that is surrounded by several layers of polymeric materials including a first or inner semiconducting shield layer (conductor or strand shield), an insulation layer, a second or outer semiconducting shield layer (insulation shield), a metallic tape or wire shield, and a protective jacket. The outer semiconducting shield can be either bonded to the insulation or strippable, with most applications using strippable shields. The inner semiconducting shield is generally bonded to the insulation layer. Additional layers within this construction such as moisture impervious materials are often incorporated.
Polymeric semiconducting shields have been utilized in multilayered power cable construction for many decades. Generally, they are used to fabricate solid dielectric power cables rated for voltages greater than 1 kiloVolt (kV). These shields are used to provide layers of intermediate conductivity between the high potential conductor and the primary insulation, and between the primary insulation and the ground or neutral potential. The volume resistivity of these semiconducting materials is typically in the range of 10−1 to 108 ohm-cm when measured on a completed power cable construction using the methods described in ICEA S-66-524, section 6.12, or IEC 60502-2 (1997), Annex C. Typical strippable shield compositions contain a polyolefin such as ethylene/vinyl acetate copolymer with a high vinyl acetate content, conductive carbon black, an organic peroxide crosslinking agent, and other conventional additives such as a nitrile rubber, which functions as a strip force reduction aid, processing aids, and antioxidants. These compositions are usually prepared in pellet form. Polyolefin formulations such as these are disclosed in U.S. Pat. No. 4,286,023 and European Patent Application 420 271.
Insulated electrical conductors are typically manufactured by coextrusion by which three layers, the inner semiconducting layer, the crosslinkable polyolefin insulation layer, and the insulation shield are extruded simultaneously, employing coaxial extruders, and subsequently cured in a single operation. This method of manufacture is advantageous in that it results in the close bonding of the three layers, eliminating partial delamination and void formation between layers, caused, during normal use, by flexure and heat. This, in turn, helps prevent premature cable failure. On the other hand, such a method of manufacture for cable constructions requiring a strippable insulation shield presents problems of strippability due to the high bond strength between the crosslinked polyolefin insulation layer and the insulation shield, caused in part by formation of crosslinking bonds across their interface.
While it is important that the insulation shield adhere to the insulation layer, it is also important that the insulation shield can be stripped with relative ease in a short period of time. It is found that the typical insulation shield does not have optimum strippability with respect to the insulation layer. Strippability is very important in that it is not only time saving, but enhances the quality of the splice or terminal connection. It is well understood by those skilled in the art, however, that thermal stability is not to be sacrificed to achieve optimum strippablity.
Three approaches have been taken to achieve acceptable strippability and thermal stability of the insulation shield in combination with commercial insulation layers made up of crosslinked polyethylene; tree retardant, crosslinked polyethylene; or ethylene/propylene copolymer rubbers. The first approach provides an insulation shield made up of an ethylene/vinyl acetate copolymer typically containing 33 percent by weight vinyl acetate and an acrylonitrile/butadiene rubber (NBR). The second uses an ethylene/vinyl acetate copolymer typically containing 40 percent or more by weight vinyl acetate and no NBR. These two approach es provide acceptable strippability, but poor thermal stability. The third approach uses an ethylene/ethyl acrylate copolymer insulation shield. This approach solves the problem of poor thermal stability, but unfortunately exhibits poor or no strippability.